This invention relates to new, water-soluble or water-dispersible resinous carboxylic acid salts, to aqueous compositions containing them, and to surfaces coated with such compositions.
To obtain a corrosion-resistant coating for metal containers which contact foodstuffs, beverages, or corrosive materials, it is conventional practice to coat the metal surface with a crosslinkable resin formulation dissolved in an organic solvent, then to heat the coating to evaporate the solvent and cross-link the resin. Cross-linking of the coating converts it into a tough, adherent, flexible, and protective film. During heating, the solvent is usually evaporated into the atmosphere. Since organic solvents are relatively expensive, inflammable, and usually environmentally objectionable, there exists a need for coatings which may be applied using minimal proportions of such solvents, particularly useful being coating compositions containing a high proportion of water. A further important requirement of such coatings is that they are themselves non-toxic and that they do not give rise to the leaching of toxic or flavour-tainting material into the foodstuff or beverage with which they may be in contact for prolonged periods. This latter requirement has in the past restricted the use of some aqueous systems.
We have now found that stable, aqueous, coating compositions which cure to give coatings having excellent mechanical properties and chemical resistance, and which in general do not give rise to the leaching of toxic or flavour-tainting material into foodstuffs or beverages, may be prepared from new resinous carboxylic salts. These salts are prepared from a phenol-terminated resin by a Mannich reaction with an aminoacid and an aldehyde, with partial or complete neutralisation of the carboxylic acid group or groups introduced by the aminoacid. They may be used with an aminoplast, a phenol-formaldehyde resin, or a blocked polyisocyanate as aqueous surface coating compositions. In certain circumstances, the addition of such a coreactant is not, however, necessary.
The use of a Mannich reaction to obtain water-soluble coating compositions has already been disclosed.
For example, in U.S. Pat. Nos. 4,188,312 and 4,189,450 there are described coating compositions containing the reaction product of an epoxide resin (i.e., a compound having, per average molecule, more than one 1,2-epoxide group) and a Mannich base and, in the first of these Patents, an unsaturated alkylphenol. This Mannich base is the reaction product of a polyhydric polynuclear phenol, a secondary amine containing at least one hydroxyalkyl group, such as an N-alkylethanolamine or an N-alkylisopropanolamine, formaldehyde and, in the second of these Patents, a butadiene or isoprene homo- or co-polymer containing phenolic groups.
British Patent Specification No. 1,428,835 describes a cationic polymer prepared by the aminomethylation of a p-hydroxystyrene polymer, using formaldehyde and a secondary amine which has alkyl, alkenyl, or aromatic groups attached to the nitrogen atom, or a secondary amine which is a heterocycle having 4 to 6 carbon atoms in the ring. The product may be quaternised or an acid addition salt may be formed from it.
U.S. Pat. Nos. 4,000,116 and 4,014,955 describe crosslinked macromolecular polyethers prepared by reaction of a Mannich base with a polyepoxide, followed by polyquaternisation, or by reaction with a compound containing both a chlorine, bromine, or iodine atom bonded to a hydrocarbon radical and at least one epoxide group. The Mannich bases are prepared from a monomeric bisphenol and the amines used are dialkylamines or morpholine.
British Patent Specification No. 1,457,932 describes epoxide group-free polymers which are water-soluble in their protonated form and are suitable for use in coatings. They are prepared by reaction of a Mannich base with an epoxide resin, followed by salt formation. The Mannich base, containing one or more cationic hydroxyalkylaminomethyl groups attached to each aromatic ring, is prepared by reaction of a bisphenol, a hydroxyl group-containing secondary amine, or a mixture of such an amine with another primary or secondary amine, and formaldehyde. The resin therefore contains these cationic groups attached to each ring in as many residues derived from the Mannich base as are in the molecular chain.
In the processes described in other patent specifications of this type, polymeric bisphenols which are reaction products of monomeric bisphenols and epoxide resins are reacted with formaldehyde and a hydroxyalkyl group-containing secondary amine to produce polymeric Mannich base bisphenols. For example, in the process described in U.S. Pat. No. 3,994,989 such a polymeric bisphenol is reacted in admixture with a monomeric bisphenol; in the process described in British Patent Specification No. 1,563,917 the polymeric bisphenol is reacted alone or in admixture with a monomeric bisphenol. The polymeric Mannich base bisphenols have cationic hydroxyalkylaminomethyl groups attached to only one aromatic ring at each end of the chain. However, these polymeric Mannich base bisphenols are not used directly in coating compositions, but are further advanced by reaction with epoxide resins to produce bisphenols of yet higher molecular weight. In the latter, the cationic groups would be attached to the terminal aromatic rings of the residues derived from the polymeric Mannich base bisphenols not only at the ends of the molecular chain of the final product but also wherever those residues occur in the remainder of the chain.
In none of the above specifications is there suggested the use of a Mannich base prepared from an aminoacid, yielding a water-soluble or water-dispersible product bearing anionic groups.
In British Patent Specification No. 1,327,071 there are described aqueous cationic electrodepositable compositions comprising the reaction product of a polyepoxide and an amine, which term is used to include aminoacids. There is no suggestion made in the Specification that the amine could be made to undergo a Mannich reaction in order to solubilise the polyepoxide.
Phenol group-containing resins having carboxy-substituted alkylaminomethyl groups, said to be useful as dispersing agents for proteins and as ingredients in adhesives, are described in U.S. Pat. No. 2,717,263. These dispersing agents are prepared by condensation of an A-stage phenolic resin with an aminoacid and formaldehyde. There is no indication whether these products would be suitable for use as coating resins, nor whether the phenolic resin could be replaced by the reaction product of a phenol with a polyepoxide.
In U.S. Pat. No. 3,936,399 phenolic chelating resins are described which are prepared by a Mannich reaction of a phenol, iminodiacetic acid, and an aldehyde, and polycondensing the resultant product with an aldehyde. The polycondensed resin has highly selective adsorbability to heavy metals. There is no indication that the intermediate material, prepared by the Mannich reaction, would have any other utility.
Finally, in U.S. Pat. No. 3,563,926, a process is described in which a resinous epoxide is condensed with a phenol or an amine, and the product is reacted with an aldehyde to give a `precondensate`. The precondensate is then reacted with a polycarboxylic acid and rendered water-soluble by the addition of a nitrogen base. As well as the use of phenols and amines, the patent also describes the use of condensates of an aldehyde with a phenol or an amine. It does not, however, describe the use of a precondensate prepared from a resinous epoxide, a phenol, an amine, and an aldehyde, and so there is no possibility of a Mannich reaction taking place.